The number of publications on this topic has not decreased for several years. This is explained by the rapid development of RFID technology itself, the development of new frequency ranges, and the regular emergence of new products and applications where contactless identification technology (or radio frequency identification) allows solving problems previously beyond the capabilities of hardware and software.

The number of publications on this topic has not decreased for several years. This is explained by the rapid development of RFID technology itself, the development of new frequency ranges, the regular emergence of new products and new applications, where contactless identification technology (or radio frequency identification - Radio Frequency IDentification) allows solving problems previously insurmountable for technical and software tools.

From Chaos to Order, or the History of the Issue

Radio frequency identification technology emerged about 20 years ago and throughout this period has developed at a pace that outpaces computer technology. RFID has improved especially intensively in the last 5-7 years. This can be explained by two factors: firstly, the development of microelectronics has made it possible to implement many ideas that were previously inaccessible for purely technological reasons, and secondly, the emergence of standards, the application of which ensured the compatibility of technical solutions from different manufacturers. Before considering specific issues of using contactless identification in various areas of human activity, let us dwell on the general principles of RFID systems and regulatory documents that determine and will determine the course of design thinking in the near future.

Technology Basics

For those who are not familiar with RFID technology, we will briefly outline its essence. The physical principles (at least for most frequency ranges) resemble the operation of a transformer or a system of coupled circuits. As is known, if you take two coils and place them not very far from each other, they will have a mutual influence on each other (Fig. 1).

Fig. 1. Operating principle of the "reader-identifier" pair

  The reader contains a high-frequency generator G, which powers the reader antenna Lc. Due to the electromagnetic coupling M between the reader antenna and the identifier (card) antenna LK, an alternating voltage is induced in the latter, the magnitude of which depends on the design and the distance between the card and the reader. The induced voltage is used to power the card microcircuit DK through a rectifier formed by diode VDп and filter capacitor Сф. The card microcircuit DK modulates the voltage in antenna IK by shunting it with resistor Кш. Due to the coupling of the antennas, modulation appears in the reader antenna Lc, is detected by diode VDд and goes to the reader microcircuit Dc, which decrypts the card code and feeds it to the controller via interface Int. The first passive R/O (Read Only) proximity cards and readers worked on this principle. Then, identifiers were created that were capable of not only transmitting information to the reader, but also receiving it for programming purposes (writing information into non-volatile memory). From the point of view of the basic principles of constructing an RFID system, a modulator appeared in the reader, which modulated the carrier emitted by the reader, and a detector and reprogrammable non-volatile memory appeared in the card, into which the information transmitted by the reader was written (Fig. 2). Identifiers (cards) with this technology are already called R/W (Read Write), that is, "reading and writing".

Fig. 2. RFID system with Read/Write technology

The first industrial RFID systems were based in the 125 kHz frequency range. But with the growing need for the volume of information transmitted in a short time, higher-frequency systems were developed, in particular, operating in the 13.56 MHz range.

Regardless of the frequency range and coding method, the design of cards operating using RFID technology is approximately the same, as shown in Fig. 3.

Fig. 3. Proximity Card Design

The operating principle of the "card - reader" pair clearly leads to the following conclusion: the greater the reading range we want to ensure, the larger the reader will be and the higher the emitted power should be. For a rough estimate of the potential range of a passive RFID system in the 125 kHz and 13.56 MHz ranges, we can take as a basis the fact that the maximum reading range of a card code is equal to the diagonal of the reader antenna. If anyone tries to convince you otherwise, don't believe them!

Frequencies and Standards

To understand all the following material meaningfully, we should consider the frequency ranges of RFID systems and the main standards that govern virtually all modern developments in this area. Let's start with frequencies. Today, RFID has occupied four frequency ranges: 125 kHz, 13.56 MHz, 800-900 MHz, and 2.45 GHz. It's worth noting that the 800-900 MHz range is used much less frequently than the other three, so we won't discuss it in more detail.

Why were these frequency ranges chosen? Because these are the frequencies that fill the gaps in the currently overcrowded frequency schedules for a wide variety of military and broadcast communications systems. In fact, these are the frequencies for which commercial development is permitted in most countries without obtaining frequency permits. For example, the 2.45 GHz range is the frequency used by Bluetooth and Wireless LAN, which are wireless networks used for consumer use. Naturally, each frequency range has its own specific RFID system characteristics, which are best illustrated by the graphs shown in Fig. 4.

Fig. 4. Dependence of RFID system parameters on frequency

Consequently, each range uses its own methods of encoding signals in the "reader - card" pair, its own transmission rates and collision resolution algorithms. The anti-collision mechanism is used so that when several identifiers are simultaneously in the reader field, it is possible to select for dialogue only one, which is needed at a given time. 

In older Proximity systems without such a mechanism, simultaneous presentation of two or more cards to the reader resulted in none of them being read. As we continue to explore, it will become clear that many modern RFID-based applications simply could not function without this tool.

But let's return to standards, as unification and standardization have always been the driving forces that allowed private solutions to be integrated into the global economy. It should be noted right away that standardization is not an event, but a process that occurs in parallel with technological development. However, once established, standards remain in effect for quite a long time (to be fair, after a certain point, they unfortunately also become a brake on progress).

So, each of the aforementioned frequency ranges has its own standards, each with its own level of development. Their most general characteristics are more conveniently presented in tabular form (see table).
 

The table does not mention the 800...900 MHz range because it is used quite rarely and the author is not aware of the standards in force for this range.

"Non-standard" solutions

Paradoxically, there are a huge number of proximity cards in circulation today that do not comply with any of the standards considered. They were simply developed and put into circulation before standardization touched the RFID area. Nevertheless, these cards still occupy the main positions in access control systems (ACS), so we will briefly dwell on their characteristics. We will immediately note that almost all of them operate in the good old 125 kHz range, for which the technical implementation was quite accessible even 15 years ago.

The solutions discussed below, which are “non-standard” by today’s approaches, have been and partly remain “de facto” standards for many years.

Indala Cards

Indala (a division of Motorola) - Historically, one of the first serial manufacturers of proximity cards and readers for access control systems.

Cards have a fixed internal card code length of 35 bits, while 26-bit readers "cut off" the excess part of the card code when converting to Wiegand format, while readers with a longer code length, for example, Wiegand 44 (also known as AMicro) "dilute" the output code with bits that have a constant value. The type (size) of the output code for Indala is determined by the reader. Indala identifiers use amplitude modulation of the carrier, divided in half, and the circuit implementation of the demodulator in the reader for them is one of the most complex.

HID Cards

Unlike Indala, any HID reader works in all forms